I gotcher DT300i with and without 4-way flashers, I gotcher DT300i with and without ABS and I gotcher DT300i with the light switch but not the US model with no light switch. Whad'll ya have?
Actually, I have a couple of wiring diagrams for the DT300i spread over several devices. Being a device dinasaur it will take a while to find ONE and then to figger out how to post here. I think I understand what you are trying to do.
The simplest way seems replace the single throw start relay with a double throw and wire from the NC side to power the low beam relay. Lemme look for that wiring diagram....
This is the one with 4-way flashers, no ABS and light switch. All other aspects seem correct including most of the wire colors. Be on alert for differences! Always!